As is known, semiconductor integrated circuit devices (“chips”) can be tested while they are still present on the semiconductor wafer on which they were formed. Such wafer level testing is traditionally accomplished on a per chip basis, in which probe tips are brought into contact with bond pads for a given chip, the chip is stimulated and tested through the probe tips via a tester, and then the wafer is indexed and moved to the next chip which is similarly tested, etc.
However, systems also exist which are capable of testing an entire semiconductor wafer, i.e., all chips on the wafer simultaneously and without the need to index from chip to chip. While such systems can be used to test the chips on the wafer for basic functionality, they are also particularly useful to stress the chips for a limited period of time for the purpose of weeding out early latent failures, what is known in the art as “burn in.” When burning in a wafer, and as shown in FIG. 1A, signals are sent from a tester 10 through a cable conduit 12 and edge connector 14 to a burn in board or “fan out” board 20. As one skilled in the art will appreciate, the burn in board 20, which typically constitutes a Printed Circuit Board (PCB), contains contact points 22 on its underside (shown in phantom) which match the locations of the bond pads 42 on the wafer 40 being tested or burned in. (Only several corresponding contact point 22/bond pad 42 pairs are shown in FIG. 1A for simplicity, although one skilled in the art will recognize that tens of thousands or more can be present). Such bond pads 42 may constitute redistribution bond pads routed to convenient locations and at a more relaxed pitch that is easier to probe than are traditional bond pads.
To electrically and physically mate the contact points 22 with the bond pads 42, an intermediary structure is interposed therebetween. This intermediary structure is sometimes known as a “Z-block,” which is so named because it mates the contact point 22/bond pad 42 pairs along the Z (or vertical) axis. The Z-block 30 contains holes 45 therethrough which similarly match the location of each contact point 22/bond pad 42 pair. Interposed in each hole is a conductive probe pin 32. When the burn in board 20, the Z-block 30, and the wafer 40 are sandwiched together during a testing or burn-in operation, and as shown in cross-section in FIG. 1B, the ends of the probe pins 32 will contact the contact points 22 on one side of the Z-block 30 and the bond pads 42 on the wafer 40 on the other side. Because of the mechanical nature of the coupling of this stack, it is preferred that the probe pins 32 be deformable in a spring like manner. Many shapes are possible for the probe pins 32, which may comprise helical spring, leaf springs, “pogo pins,” springs made from flat sheets of metal, etc.; the S-shaped probe pin shown in the Figures is merely exemplary, and other probe pin designs will be discussed in this disclosure.
It can be difficult to construct a Z-block 30, as a number of parameters constrain its design. For example, the Z-block must capture the probe pins 32 so that the pins will not fall through the Z-block. This usually requires the use of a probe pin 32 having two effective diameters: an end diameter D1 and a body diameter D2, as shown in FIG. 2A. In this example, the probe pin 32 is essentially a helical spring with straight ends used to contact the contact points 22/bond pads 42, although other pin shapes are possible. To accommodate and capture such a probe pin 32, the Z-block 30 must in turn also contain through holes 45 with two different diameters: one diameter D3 sufficiently large for the effective diameter of the probe pin end (D1) to fit through, but smaller than the effective diameter of the probe pin body diameter (D2); and a second diameter D4 sufficiently large to accompany the probe pin body diameter (D2).
This relationship can be accomplished in different ways. For example, and as shown in FIG. 2A, the Z-block 30 can be formed of two separate pieces 50a and b. Each piece has holes 45 drilled therein corresponding to the eventual location of the probe pins 32. Through the use of an angled drill bit, holes 45 having a conical ends can be formed which meet the two-diameter requirement noted above for retaining the pins 32. Such holes 45 must be drilled precisely along the Z-axis lest the diameters of the holes (e.g., D3) become skewed or non-uniform, which can be very difficult or expensive to accomplish. Ultimately, the probe pins 32 are placed within the holes 45 in the lower of the two pieces 50a, and thereafter the upper piece 50b is affixed (e.g., bolted, glued, etc.) to the lower piece 50a to capture the probe pins 32, as shown in FIG. 2B. FIGS. 2C and 2D disclose other Z-block geometries for capturing the probe pins 32 which again use multiple affixable pieces, although in these examples the pieces are drilled with perfectly cylindrical holes 45.
Regardless of the scheme used to form the Z-block 32, ultimately all of the schemes involve the same step of (1) placing the pins 32 into holes 45 in at least one lower piece of the Z-block, and (2) placing at least one other piece of the Z-block over the pins to capture them and affixing the pieces together. But this is difficult to do in practice. The probe pins 32 are very delicate, being on the order of 100 mils in length and made of wire which may have a thickness of approximately 3 mils for a 100 mil length coil spring pin. It is therefore difficult to insert the probe pins 32 into the lower piece 50b such that their ends protrude through the smaller diameter holes (D3). Moreover, if the probe pins 32 lay askew in their holes 45 when the upper piece 50b is placed on top of the lower piece 50a, the ends of the pins 32 may not pass through the holes 45 in the upper piece, and instead will become bent within the holes and unable to make contact with the points/pads 22 or 42. This problem is exacerbated when it is recognized that a typical Z-block 30 may contain tens of thousands of probe pins 32, thus requiring the ends of the pins to simultaneously pass through the upper piece 50b when it is mounted to the lower piece 50a, a formidable challenge. Damage to any one of these pins 32 may require opening and re-working the Z-block 30 to replace affected or damages pins, which requires unaffixing (e.g., unbolting) the upper and lower pieces 50a and 50b. This reopening procedure too can cause problems, as this operation can tend to drag the otherwise properly-aligned pins 32 out of their holes 45, which can occur if the delicate pins bind to the smaller diameter portions (D3) of the holes. This can be a problem even if no pins 32 are damaged through assembly of the Z-block, but instead become worn though use and need replacement.
Moreover, capturing the spring limits the sorts of probe pins that can be used with known prior art Z-block approaches. As noted above, the probe pins 32, be they springs, pogo pins, etc., necessarily must contain end portions with smaller effective diameters (D1) than the main body portion of the pins (D2) so that they can be captured, but still protrude from, the holes 45 (D3, D4). Such a limitation to the design of the probe tips is unfortunate. For example, consider the probe pin 32 of FIG. 3, which has a flat, long end 32a. This end design for the pin may be used, for example, as the portion of the pin 32 to make contact with the bond pads 42, which is beneficial as a flat end 32a is less likely to stab and damage the bond pads 42. However, such a pin design cannot be used with prior art means for capturing the springs as shown in FIGS. 2A–2D, because the effective diameter of end 32a (D5) is greater than the effective diameter of the body of the spring (D6).
Another problem that prior art Z-block designs do not adequately address is the issue of probe pin orientation within the holes of the Z-block. As shown by the probe pin design of FIG. 3, not all probe pins are rotatably symmetrical about their long axis 60. But probe pins 32, when placed in the holes 45 in prior art Z-blocks, will be able to rotate freely within the holes 45. This is undesirable, especially for asymmetric probe pins. For example, as shown in FIG. 4, if the probe pin of FIG. 3 was mounted within a prior art Z-block, it would be allowed to turn such that the various ends 32a of the probe pins 32 would become misaligned (in solid lines). Such non-uniformity is undesirable as it may make contact of such ends to bond pads 42/contact points 22 unreliable or could result in shorting and limiting probe design. It would therefore be preferable for the probe pins 32 to be non-movably aligned in the holes, which would, for example, allow the ends 32a to have a uniform orientation (in dashed lines).
In short, there is room to improve to Z-block designs. Such improvement would preferably make assembly of the Z-block easier, installing and servicing of the probe pins easier, would accommodate a wide variety of probe pin designs, and would be cheaper to manufacture. This disclosure presents such solutions.